Synchrotron-based X-ray Photoelectron Spectroscopy Research Articles

Ambient pressure x-ray photoelectron spectroscopy study on the initial atomic layer deposition process of platinum

The initial adsorption of MeCpPtMe3 is investigated using synchrotron-based ambient pressure x-ray photoelectron spectroscopy (XPS). The experiments are done on a native oxide-covered Si substrate. In addition, a reaction with O2 and the created Pt surface was investigated. Inspiration for the reaction studies was found from atomic layer deposition of metallic Pt, process that uses the same compounds as precursors. With time-resolved XPS, we have been able to observe details of the deposition process and especially see chemical changes on the Pt atoms during the initial deposition of the Pt precursor. The change of the binding energy of the Pt 4f core level appears to occur on a different timescale than the growth of the active surface sites. The very long pulse of the Pt precursor resulted in a metallic surface already from the beginning, which suggest chemical vapor deposition-like reactions occurring between the surface and the precursor molecules in this experiment. Additionally, based on the XPS data measured after the Pt precursor pulse, we can make suggestions for the reaction pathway, which point toward a scenario that leaves carbon from the MeCpPtMe3 precursor on the surface. These carbon species are then efficiently removed by the subsequent coreactant pulse, leaving behind a mostly metallic Pt film.

Understanding the morphology and chemical activity of model ZrOx/Au (111) catalysts for CO2 hydrogenation

In this study, the growth of ZrOx on Au (111) was investigated using scanning tunneling microscopy (STM) and synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Nanostructures of ZrOx (x = 1,2) at the sub-monolayer (≤ 0.3 ML) level were prepared by vapor depositing Zr metal onto Au (111) followed by oxidation with O2 or CO2. At low coverages of the admetal (< 0.05 ML), the formed ZrOx nanostructures were dispersed randomly on the terraces and steps of the Au(111) substrate. Strong oxide-metal interactions prevented the formation of islands of zirconia. The ZrOx nanostructures displayed a reactivity towards CO2 and H2 not seen for bulk zirconia. C 1 s AP-XPS results indicated that CO2 molecules adsorbed on Zr/ZrOx/Au(111) surfaces could undergo partial decomposition on Zr (CO2, gas → COgas + Oads), or react with oxygen sites from ZrOx to yield carbonates (Zr-CO3, ads). After exposing ZrO2/Au (111) surfaces to 1:3 mixtures of CO2:H2, the formation of HCOO, CO3, and CH3O was detected in AP-XP spectra. These chemical species decomposed at temperatures in the range of 400‒600 K, making them possible reaction intermediates for methanol synthesis.

Approaching the Multiphase Interface of Ir-Based PEM Electrolyzers Using Operando ‘Tender’ X-Ray AP-XPS

Electrochemical water splitting is a promising pathway for the renewable production of hydrogen as an alternative fuel and chemical feedstock. Yet, the process is plagued by slow kinetics of the essential oxygen evolution reaction at the anode. Further, large-scale hydrogen production from such devices will require reduced platinum group metal (PGM) loadings (often iridium-based anodes) and/or new materials due to factors such as PGM scarcity, cost and loss during operation. In the hopes of enabling progress of rational design for OER catalysts, effective characterization of the electrode-liquid interface is crucial. Synchrotron-based X-ray Photoelectron Spectroscopy (XPS) offers a versatile way to obtain surface chemical and elemental characterization of these materials. The use of X-rays in the ‘tender’ regime (~2-6 keV) allows for deeper probing than the conventional ‘soft’ x-ray (< 2000 eV) set up that is common at synchrotrons and lab-based sources. The photoelectrons generated from tender x-rays have enough energy to penetrate tens of nanometers through low density components such as a layer of liquid or polymeric electrolyte. This opens the door to access the chemistry at the interface of liquid and ionomer electrolyte-electrocatalyst materials. With the use of a fully operational two-electrode system, we are able to study these membrane electrode assemblies under active water splitting conditions.My talk will explain our specific approach, which is often crucial for successful collection of accurate data, highlighting elements of cell design and experimental setup that are helpful for this process. I will also show recent AP-XPS results on iridium-based polymer electrolyte membranes for water splitting. With application of elevated potential, we can correlate the iridium valency and surface species with the appearance of product traces in a mass spectrum. Figure 1

Synthesis of B-site high-entropy BaTi0.2Hf0.2Zr0.2Y0.2Nb0.2O3 with water sensing properties

A barium-based perovskite structured ceramic with five different B-site cations was prepared by a solid-state synthesis method. The phase and chemical homogeneity of the synthesised material was verified using x-ray diffraction and energy dispersive x-ray spectroscopy, showing the successful synthesis of a single-phase perovskite structure with Ba A-site and Ti, Hf, Zr, Y, Nb, B-site cations. Ultra-high vacuum x-ray photoelectron spectroscopy was used to reveal the valence states of the constituent elements. Conventional, two-step and spark plasma sintering were used to form dense pellets with limited grain growth. The room temperature electrical characteristics of the sintered pellets were investigated by electrochemical impedance spectroscopy where a conductivity increase of two orders of magnitude was observed in a water-bearing atmosphere. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy performed under water-bearing and dry conditions suggest that the conductivity increase is related to the incorporation of hydroxyl groups into the perovskite structure.

The Surface Chemistry of Methanol on Pd(111) and H-Pd(111) Surfaces: C-O Bond Cleavage and the Effects of Metal Hydride Formation.

Palladium catalysts are frequently employed in processes where methanol is an energy vector or carrier, being useful for the synthesis of methanol from mixtures of carbon dioxide and hydrogen (CO2/H2) or its steam reforming on demand. Results of synchrotron-based ambient pressure X-ray photoelectron spectroscopy for the adsorption of methanol on a Pd(111) model catalyst show a rich surface chemistry and complex phenomena that strongly depend on pressure and temperature. At low pressures (<10-6 Torr) and temperatures (<300 K), CO is the dominant decomposition product. As the pressure increases, cleavage of C-H, O-H, and C-O bonds is observed, and at elevated temperatures (400-600 K) the formation of CO and CHx/C fragments compete on the surface. Thus, existing reaction networks for methanol decomposition must be modified. Furthermore, surface and subsurface hydrogen (coming from PdHx) play a significant role in the stability and removal of CHx and C species.

Construction of an Axial Charge Transfer Channel Between Single-Atom Fe Sites and Nitrogen-Doped Carbon Supports for Boosting Oxygen Reduction.

The introduction of axial-coordinated heteroatoms in Fe─N─C single-atom catalysts enables the significant enhancement of their oxygen reduction reaction (ORR) performance. However, the interaction relationship between the axial-coordinated heteroatoms and their carbon supports is still unclear. In this work, a gas phase surface treatment method is proposed to prepare a series of X─Fe─N─C (X = O, P, and S) single-atom catalysts with axial X-coordination on graphitic-N-rich carbon supports. Synchrotron-based X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy indicate the formation of an axial charge transfer channel between the graphitic-N-rich carbon supports and single-atom Fe sites by axial O atoms in O─Fe─N─C. As a result, the O─Fe─N─C exhibits excellent ORR performance with a half-wave potential of 0.905V versus RHE and a high specific capacity of 884 mAh g-1 for zinc-air battery, which is superior to other X─Fe─N─C catalysts without axial charge transfer and the commercial Pt/C catalyst. This work not only demonstrates a general synthesis strategy for the preparation of single-atom catalysts with axial-coordinated heteroatoms, but also presents insights into the interaction between single-atom active sites and doped carbon supports.

Interface defect formation for atomic layer deposition of SnO2 on metal halide perovskites

With the rapidly advancing perovskite solar cell (PSC) technology, dedicated interface engineering is critical for improving device stability. Atomic layer deposition (ALD) grown metal oxide films have drawn immense attention for the fabrication of stable PSC. Despite the advantages of ALD, the deposition of metal oxides directly on bare perovskite has so far not been achieved without damaging the perovskite layer underneath. In addition, the changes to the physicochemical and electronic properties at the perovskite interface upon exposure to the ALD precursors can alter the material and hence device functionality. Herein, we report on a synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES) investigation of the interface between metal halide perovskite (MHP) absorber and ALD-SnO2 electron transport layer. We find clear evidence for the formation of new chemical species (nitrogen compound, lead dihalides) and an upward band bending in the MHP and downward band bending in the SnO2 towards the MHP/ALD-SnO2 interface. The upward bending at the interface forms an electron barrier layer of ∼400 meV, which is detrimental to the PSC performance. In addition, we assess the effectiveness of introducing a thin interlayer of the organic electron transport material Phenyl-C61-butyric acid methyl ester (PCBM) between MHP and ALD-SnO2 to mitigate the effects of ALD deposition.

Investigating structural, optical, and electron-transport properties of lithium intercalated few-layer MoS2 films: Unraveling the influence of disorder

Molybdenum disulfide is a promising candidate for various applications in electronics, optoelectronics, or alkali-ion batteries. The natural presence of the van der Waals gap allows intercalating alkali ions, such as lithium, into MoS2 films. Intercalation can modify the electronic structure as well as the electrical and optical properties. Here, we present a structural, optical, and electrical characterization of Li-intercalated few-layer MoS2 films. The intercalation was carried out by annealing MoS2 film in the presence of Li2S powder, serving as a lithium source. The initial MoS2 layers were prepared by pulsed laser deposition (PLD) and by sulfurization of 1 nm thick Mo film (TAC). The presence of lithium was confirmed by synchrotron-based x-ray Photoelectron Spectroscopy. The Raman spectroscopy, x-ray diffraction, and optical absorption measurements confirmed semiconducting behavior for all samples. All samples exhibited the thermally activated dependence of the electrical resistance, R, typical for the Efros–Shklovskii variable range hopping in a disordered semiconductor, ln R(T) ∝ (TES/T)1/2, where kBTES is the hopping activation energy. The PLD-grown MoS2 samples exhibited a relatively mild initial disorder primarily caused by grain boundaries. Lithium intercalation led to an increase in disorder, evident in the increase in kBTES and a substantial rise in electrical resistance. The TAC-grown undoped MoS2 sample already exhibited significant resistance, and the impact of Li intercalation on resistance was minimal. This observation was attributed to the fact that the TAC-grown MoS2 samples exhibit a perturbed stoichiometry (the S:Mo ratio ∼ 2.20), causing strong disorder even before Li intercalation. The electron doping caused by lithium, if any, was completely obscured by the effect of disorder.

Tungsten Oxide-Based Materials as Catalyst Support for Oxygen Evolution Reaction

The high cost of iridium (Ir) is a major concern for the large scale deployment of polymer electrolyte membrane water electrolysis (PEMWE). To mitigate its impact, the usage of Ir on the anode must be significantly reduced. Down-sizing the catalyst particle is a straight forward strategy but requires the use of appropriate support materials to disperse and anchor fine Ir nanoparticles. We have investigated the use of tungsten oxide-based materials as support for the oxygen evolution reaction (OER) Ir and ruthenium (Ru) catalyst. The synthesis of nanostructured substoichiometric tungsten oxide and the subsequent loading of ultrafine Ir/Ru-based nanoparticles is thoroughly studied. Due to the successful loading, the mass activity of the tungsten oxide-supported Ir/Ru-based catalyst shows significant improvements under rotating disk electrode (RDE) conditions compared with commercially available Ir/Ru-blacks. Furthermore, non-destructive depth profiling by synchrotron-based X-ray photoelectron spectroscopy (XPS) has revealed the strong catalyst-support interaction which suppressed the oxidation of the supported Ir/Ru-based catalysts, leading to much enhanced durability under accelerated durability testing conditions. To verify its performance under practical PEM electrolysis conditions, the tungsten oxide-supported OER catalyst is directly coated onto a polymer electrolyte membrane, i.e., forming a catalyst coated membrane (CCM), and tested under single cell testing conditions without pressure differential. The voltage dependent chemical state of the supported catalyst is also probed in-situ by synchrotron-based XPS where a potential dependent electron transfer between the catalyst and tungsten-oxide support is discovered. Our discovery is anticipated to aid the development of cost effective PEM electrolysis anodes.

Fundamental Understanding and Quantification of Capacity Losses Involving the Negative Electrode in Sodium-Ion Batteries.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (1 H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF6 in ethylene carboante mixed with diethylene carbonate electrolyte(i.e., 5 µAhh-1/2 pause or 2.78 mAhg·h-1/2 pause ) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAhh-1/2 pause or 0.72 mAhg·h-1/2 pause ).

Novel NASICON-Type Na-V-Mn-Ni-Containing Cathodes for High-Rate and Long-Life SIBs.

Partial substitution of V by other transition metals in Na3 V2 (PO4 )3 (NVP) can improve the electrochemical performance of NVP as a cathode for sodium-ion batteries (SIBs). Herein, phosphate Na-V-Mn-Ni-containing composites based on NASICON (Natrium Super Ionic Conductor)-type structure have been fabricated by sol-gel method. The synchrotron-based X-ray study, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) studies show that manganese/nickel combinations successfully substitute the vanadium in its site within certain limits. Among the received samples, composite based on Na3.83 V1.17 Mn0.58 Ni0.25 (PO4 )3 (VMN-0.5, 108.1 mAh g-1 at 0.2 C) shows the highest electrochemical ability. The cyclic voltammetry, galvanostatic intermittent titration technique, in situ XRD, ex situ XPS, and bond valence site energy calculations exhibit the kinetic properties and the sodium storage mechanism of VMN-0.5. Moreover, VMN-0.5 electrode also exhibits excellent electrochemical performance in quasi-solid-state sodium metal batteries with PVDF-HFP quasi-solid electrolyte membranes. The presented work analyzes the advantages of VMN-0.5 and the nature of the substituted metal in relation to the electrochemical properties of the NASICON-type structure, which will facilitate further commercialization of SIBs.

Comparative Studies of Ir-Based PEM Electrolyzers Via Operando ‘Tender’ X-Ray AP-XPS

Ir-based catalysts are considered the state-of-the-art cathode material for the oxygen evolution reaction (OER) in water splitting devices. However, large scale hydrogen production from such devices will require reduced Ir loadings and/or new materials due to factors such as Ir scarcity, cost and loss during electrolysis operation. In the hopes of enabling progress of rational design for OER catalysts, effective characterization of the electrode-liquid interface is crucial. Synchrotron-based X-ray Photoelectron Spectroscopy (XPS) offers a versatile way to obtain surface chemical and elemental characterization of these materials. The use of X-rays in the ‘tender’ regime (~2-6 keV) allows for deeper probing than the conventional ‘soft’ x-ray (< 2000 eV) set up that is common at synchrotrons and lab-based sources. The photoelectrons generated from tender x-rays have enough energy to penetrate tens of nanometers through low density components such as a layer of liquid or polymeric electrolyte. This opens the door to access the chemistry at the interface of liquid and ionomer electrolyte-electrocatalyst materials. With the use of a fully operational two-electrode system, we are able to study these membrane electrode assemblies under active water splitting conditions.My talk will highlight recent AP-XPS results on iridium-based polymer electrolyte membranes for water splitting. With application of elevated potential, we can correlate the iridium valency and surface species with the appearance of product traces in a mass spectrum. I will show trends within and between Ir catalyst types that lend to a better understanding of these OER cathodes. Figure 1

Relating Selective Electrochemical Nitrate Reduction to Electronic Structure by AP-XPS

Ammonia produced by the Haber-Bosch process fixes nitrogen by sourcing hydrogen from methane steam reforming—today’s most carbon intensive industrial process. Alternatively, ammonia can be produced electrochemically from industrial waste nitrogen sources (e.g. nitric oxides, and nitrate), sourcing protons from water and electrons as reducing agents. However, reduction of such oxidized nitrogen species involves complex reaction mechanisms where the stability of critical reaction intermediates (e.g. nitric oxide) dictates selectivity towards ammonia as a final product. Inspired by Hammer and Nørskov’s d-band theory, we utilize synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) to explore the adsorption affinity, and ability of a surface to dissociate, nitric oxide for a series of transition metals (Fe, Co, Ni, Cu). Nitric oxide adsorption affinity and dissociation are then linked to transition metal electronic structure and contextualized further with ex-situ electrochemical selectivity measurements and density functional theory (DFT) calculations.

Adsorption and Thermal Decomposition of Triphenyl Bismuth on Silicon (001).

We investigate the adsorption and thermal decomposition of triphenyl bismuth (TPB) on the silicon (001) surface using atomic-resolution scanning tunneling microscopy, synchrotron-based X-ray photoelectron spectroscopy, and density functional theory calculations. Our results show that the adsorption of TPB at room temperature creates both bismuth-silicon and phenyl-silicon bonds. Annealing above room temperature leads to increased chemical interactions between the phenyl groups and the silicon surface, followed by phenyl detachment and bismuth subsurface migration. The thermal decomposition of the carbon fragments leads to the formation of silicon carbide at the surface. This chemical understanding of the process allows for controlled bismuth introduction into the near surface of silicon and opens pathways for ultra-shallow doping approaches.

Revealing CO2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO2) activation. Despite extensive reactivity benchmarks, a correlation between CO2 conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO2(g). The CO2 dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO2 conversion to renewable energy sources during C1 chemical reactions.

Liquid Organic Hydrogen Carriers: Model Catalytic Studies on the Thermal Dehydrogenation of 1-Cyclohexylethanol on Pt(111)

The molecule pair of 1-cyclohexylethanol and acetophenone represents an interesting system for a chemical hydrogen storage cycle as it combines two different classes of so-called liquid organic hydrogen carriers (LOHCs), namely hydrogenated, formerly aromatic cyclic hydrocarbons and alcohols, i.e., hydrogenated carbonyls. In particular, the latter have recently attracted much attention due to their favorable dehydrogenation temperatures and the possibility to convert them directly into electricity in specially designed direct-LOHC fuel cells. Herein, we investigate the temperature-triggered dehydrogenation reaction of 1-cyclohexylethanol to acetophenone on a Pt(111) model catalyst using synchrotron-based temperature-programed X-ray photoelectron spectroscopy. To obtain a complete picture of the reaction mechanism, we consider not only the individual surface reactions of 1-cyclohexylethanol and acetophenone but also those of two potential dehydrogenation intermediates, namely, the partially dehydrogenated 1-cyclohexylethanone and 1-phenylethanol. We find a stepwise dehydrogenation of 1-cyclohexylethanol: the first step around ∼210 K at the alcohol moiety yields the ketone 1-cyclohexylethanone. The second step above ∼260 K at the cyclohexyl group is accompanied by the loss of an H-atom at the molecule’s methyl group and leads to the formation of an acetophenone-like phenyl–C(O)–CH2 species at ∼340 K. The same species are also identified in the surface reactions of acetophenone, 1-phenylethanol, and 1-cyclohexylethanone in this temperature range. Overall, the system shows good thermal robustness: a complete dehydrogenation of the hydrogen-rich carrier occurs without damage to the carbon framework.

Electronic Activation during Nanoparticle Exsolution for Enhanced Activity at Elevated Temperature.

Nanoparticle (NP) exsolution from perovskite-based oxides matrix upon reduction has emerged as an ideal platform for designing highly active catalysts for energy and environmental applications. However, the mechanism of how the material characteristics impacts the activity is still ambiguous. In this work, taking Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3 thin film as the model system, we demonstrate the critical impact of the exsolution process on the local surface electronic structure. Combining advanced microscopic and spectroscopic techniques, particularly scanning tunneling microscopy/spectroscopy and synchrotron-based near ambient X-ray photoelectron spectroscopy, we find that the band gaps of both the oxide matrix and exsolved NP decrease during exsolution. Such changes are attributed to the defect state within the forbidden band introduced by oxygen vacancies and the charge transfer across the NP/matrix interface. Both the electronic activations of oxide matrix and the exsolved NP phase lead to good electrocatalytic activity toward the fuel oxidation reaction at elevated temperature.

Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems.

Halogenation of organic molecules causes chemical shifts of C1s core-level binding energies that are commonly used as fingerprints to identify chemical species. Here, we use synchrotron-based X-ray photoelectron spectroscopy and density functional theory calculations to unravel such chemical shifts by examining different partially fluorinated pentacene derivatives. Core-level shifts occur even for carbon atoms distant from the fluorination positions, yielding a continuous shift of about 1.8 eV with increasing degree of fluorination for pentacenes. Since also their LUMO energies shift markedly with the degree of fluorination of the acenes, core-level shifts result in a nearly constant excitation energy of the leading π* resonance as obtained in complementary recorded K-edge X-ray absorption spectra, hence demonstrating that local fluorination affects the entire π-system, including valence and core levels. Our results thus challenge the common picture of characteristic chemical core-level energies as fingerprint signatures of fluorinated π-conjugated molecules.

Synchrotron-based near ambient-pressure X-ray photoelectron spectroscopy and electrochemical studies of passivation behavior of N- and V-containing martensitic stainless steel

Passivation behavior of a N- and V-containing martensite stainless steel was studied by synchrotron-based near ambient-pressure X-ray photoelectron spectroscopy, electrochemical analyses, and thermodynamic calculation. The passive film consists of Cr3+, Fe(2, 3)+, and V(2, 3, 4)+ oxides as inner layer, and Cr3+ and Fe3+ hydroxides as outer layer. Austenitization at 1080 oC (rather than 1010 oC) and anodic polarization facilitate transformation of CrN to Cr2O3 leading to further enrichment of Cr3+ oxide in the passive film. Whereas higher Cl- concentration promotes film dissolution leading to higher level of point defects and higher fraction of remaining V oxides in the passive film.

Synchrotron XPS and Electrochemical Study of Aging Effect on Passive Film of Ni Alloys

To investigate aging effect on the passive film of Ni23Cr15Mo and Ni22Cr9Mo3Nb, synchrotron-based X-ray photoelectron spectroscopy (XPS) was used to analyze the structure and composition of the air-formed passive film on the alloys. The corrosion resistance of the two Ni alloys in 1 M NaCl solution was evaluated with electrochemical cyclic polarization measurement. The synchrotron XPS measurement provided detailed information about chemical states of alloying elements in the passive film, showing that the passive film consists of an inner oxide layer and an outer hydroxide layer. The XPS data allowed precise determination of the chemical composition and the thickness of the outer hydroxide layer, the inner oxide layer, and the underlying subsurface alloy layer. The Cr-oxide in the inner layer grows thicker with aging time, leading to Cr-depletion in the subsurface region. Mo and Nb in the alloy form mixed oxides and hydroxides, and aging in air leads to transformation of the lower valence oxides into higher valence oxides. The freshly formed oxide film exhibits similar barrier properties as the aged oxide film. The stability of the passive film formed on Ni22Cr9Mo3Nb seems to be better than that on Ni23Cr15Mo.